1,1,1,3,3,3-Hexafluoro-2,2-di(3,4-dicarboxyphenyl) propane anhydride also known as 4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride) and hereinafter referred to as 6-FDA is a useful chemical intermediate. Polyimides made from 6-FDA have desirable properties such as low dielectric constants, low moisture uptake and the proper thermal stability making them useful as coatings and dielectrics in electronic applications. Other potential uses for polymers made with 6-FDA are composites, molded parts and optical fibers. ##STR1##
Oxidation processes for producing 6-FDA from 1,1,1,3,3,3-hexafluoro-2,2-di(3,4-dimethylphenyl) propane, also known as 4,4'-(hexafluoroisopropylidene)-bis(o-xylene) and herein after referred to as 6-FXP, are described in Coe, U.S. Pat. No. 3,310,573 where 6-FXP is first oxidized using potassium permanganate in a solvent of pyridine and water to the tetracarboxylic acid, 1,1,1,3,3,3-hexafluoro-2,2-di(3,4-dicarboxyphenyl) propane, also known as 4,4'-(hexafluoroisopropylidene)bis(phthalic acid) and hereinafter referred to as 6-FTA. The tetracarboxylic acid, 6-FTA, is then dehydrated in a separate step to form 6-FDA.
Another route to 6-FDA from 6-FXP is to utilize the known, so-called, Mid-Century Oxidation process to oxidize 6-FXP to the tetracarboxylic acid, 6-FTA. In this process the 6-FXP is oxidized to 6-FTA in the liquid phase using air or other suitable source of molecular oxygen and an oxidation catalyst comprising heavy metal components and a bromine source. Such a suitable liquid phase oxidation process is described in Saffer and Barker, U.S. Pat. No. 2,833,816. This catalytic, liquid phase oxidation reaction converts the methyl substituents on the 6-FXP molecule to carboxylic acid substituents.
In the Mid-Century-type oxidation process when used to prepare an aromatic carboxylic acid such as terephthalic acid by the oxidation of p-xylene, the terephthalic acid product is substantially insoluble in the oxidation reaction mixture or reaction product mixture and precipitates rapidly from these mixtures. This is particularly the case after the reaction product mixture is cooled to below the temperature used for the oxidation. Generally, the oxidation reaction solvent is a low molecular weight aliphatic carboxylic acid such as acetic acid. Consequently, the product aromatic carboxylic acid, such as terephthalic acid, is easily recoverable by simply separating the insoluble aromatic acid product from the oxidation reaction product mixture.
Similarly, the liquid phase, heavy-metal catalyzed oxidation of 6-FXP to the tetracarboxylic acid, 6-FTA proceeds in high yield and is an efficient method for preparing 6-FTA. However, the 6-FTA is generally soluble in the oxidation reaction product mixture and does not precipitate at all or only very slowly and cannot therefore be easily recovered from the reaction mixture to be converted into the desired dianhydride, 6-FDA.
A new process for converting 6-FXP to 6-FDA utilizing the Mid-Century-type liquid phase oxidation process and providing for the rapid and economical recovery of 6-FDA would be highly advantageous. The present invention provides such a process.